Simplify TimeSpanFormat

[lilinus]

I tried to consolidate some code in TimeSpanFormat (and minor change in DateTimeFormat).

No big performance improvements but I checked for regressions. Major gain for this PR is code is simpler, and a method in common source file is moved to CoreLib, which was the only place it was used.

Benchmarks (source)

| Method          | Job        | Toolchain | value               | fmt                                     | Mean      | Error    | StdDev   | Median    | Min       | Max       | Ratio | Gen0   | Allocated | Alloc Ratio |
|---------------- |----------- |-----------|-------------------- |---------------------------------------- |----------:|---------:|---------:|----------:|----------:|----------:|------:|-------:|----------:|------------:|
| DateTime_Format | Job-CQLODJ | main      | 01/01/0001 00:20:34 | yyyy\\-MM\\-dd\\.HH\\.mm\\.ss\\.FFFFFFF | 106.10 ns | 0.203 ns | 0.159 ns | 106.09 ns | 105.85 ns | 106.41 ns |  1.00 | 0.0306 |      64 B |        1.00 |
| DateTime_Format | Job-JQHOSH | pr        | 01/01/0001 00:20:34 | yyyy\\-MM\\-dd\\.HH\\.mm\\.ss\\.FFFFFFF |  99.43 ns | 0.342 ns | 0.303 ns |  99.46 ns |  98.96 ns |  99.90 ns |  0.94 | 0.0304 |      64 B |        1.00 |
| TimeSpan_Format | Job-CQLODJ | main      | 00:20:34.5678900    | d\\.hh\\.mm\\.ss\\.FFFFFFF              | 175.52 ns | 0.715 ns | 0.669 ns | 175.63 ns | 173.67 ns | 176.38 ns |  1.00 | 0.0265 |      56 B |        1.00 |
| TimeSpan_Format | Job-JQHOSH | pr        | 00:20:34.5678900    | d\\.hh\\.mm\\.ss\\.FFFFFFF              | 163.26 ns | 0.648 ns | 0.606 ns | 163.28 ns | 161.23 ns | 163.79 ns |  0.93 | 0.0267 |      56 B |        1.00 |
| TimeSpan_Format | Job-CQLODJ | main      | 00:20:34.5678900    | g                                       |  30.59 ns | 0.195 ns | 0.182 ns |  30.54 ns |  30.38 ns |  30.88 ns |  1.00 | 0.0228 |      48 B |        1.00 |
| TimeSpan_Format | Job-JQHOSH | pr        | 00:20:34.5678900    | g                                       |  29.94 ns | 0.105 ns | 0.098 ns |  29.91 ns |  29.79 ns |  30.09 ns |  0.98 | 0.0228 |      48 B |        1.00 |
| TimeSpan_Format | Job-CQLODJ | main      | 00:20:34.5678900    | G                                       |  31.69 ns | 0.072 ns | 0.067 ns |  31.71 ns |  31.54 ns |  31.80 ns |  1.00 | 0.0306 |      64 B |        1.00 |
| TimeSpan_Format | Job-JQHOSH | pr        | 00:20:34.5678900    | G                                       |  31.26 ns | 0.111 ns | 0.104 ns |  31.27 ns |  30.96 ns |  31.40 ns |  0.99 | 0.0305 |      64 B |        1.00 |

[lilinus]

Only usage prior to change was in TimeSpanFormat.TryFormatStandard

[lilinus]

Tried to follow pattern in DateTime.GetTime

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-datetime
See info in area-owners.md if you want to be subscribed.

[tarekgh]

@lilinus Thank you for submitting the pull request. Typically, if a code change doesn't offer significant benefits, we aim to avoid merely cleaning up or refactoring it. While this change may pass the CI tests, there's always a chance of regression. We might consider accepting it in the future if we make other modifications to the code. Thanks once more.