Remove unused scalars from SDL

[sungam3r]

According to the specification, it is not necessary to print unused types (in particular scalars).

Relates to graphql/graphql-spec#648

[sungam3r]

PR is ready for review.

Summary changes after information from graphql/graphql-spec#597 :

1. Introspection response does not return ALL unused types (which essentially means not supported types): built-in scalars, custom scalars, graph types.
2. SchemaPrinter does the same as (1.) + also does not print all built-in scalars at all.
3. SchemaPrinter.IsDefinedType can be overridden to allow printing more types. Example - AllTypesSchemaPrinter from tests.

These changes are in accordance with the specification. @joemcbride, can you do a review, please?

[sungam3r]

This PR is no longer needed after rework of GraphTypesLookup in #1887 . However, there will be one small change, maybe later in separate PR:

2. SchemaPrinter does the same as (1.) + also does not print all built-in scalars at all.

[sungam3r]

OK. As I expected, all scalars are already printed correctly thanks to our edits in other PRs.

[sungam3r]

@Shane32 PR is ready for review.

[codecov-io]

Codecov Report

Merging #1423 (3fa0bf0) into develop (c277b10) will decrease coverage by 0.01%.
The diff coverage is 96.55%.

@@             Coverage Diff             @@
##           develop    #1423      +/-   ##
===========================================
- Coverage    84.07%   84.06%   -0.02%     
===========================================
  Files          371      371              
  Lines        13098    13076      -22     
  Branches      1937     1935       -2     
===========================================
- Hits         11012    10992      -20     
+ Misses        1555     1553       -2     
  Partials       531      531              

Impacted Files	Coverage Δ
src/GraphQL/GraphQLExtensions.cs	88.88% <94.44%> (+0.67%)	⬆️
...hQL/Utilities/Federation/FederatedSchemaPrinter.cs	85.13% <100.00%> (ø)
src/GraphQL/Utilities/SchemaPrinter.cs	84.01% <100.00%> (-0.74%)	⬇️
src/GraphQL/Utilities/SchemaPrinterOptions.cs	100.00% <100.00%> (ø)
src/GraphQL/Types/Schema.cs	88.39% <0.00%> (-1.11%)	⬇️

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[Shane32]

I only have one minor comment. Please read and then merge at your discretion @sungam3r