blocklist - Don't display the current theme's template variations

[bobbingwide]

In Gutenberg 13.8.0 the oik-block/blocklist block displayed a whole load of new variations for the core/template-part block.

• These are not insertable in the Block editor.
• They are also unique for the current theme.
• therefore, in my opinion, they should not be listed in the oik-block/blocklist set of blocks.

Requirement

• The oik-block/blocklist block should not display theme specific variations of the core/template-part block.

Proposed solution

• For the core/template-part block type filter out variations which have attributes.theme set
• This logic is also needed when displaying variations in the blockinfo block.

[bobbingwide]

The filtering logic needs to be the same for both the block list and the block info block.
In the first attempt the logic was duplicated but only checked the block type for the block list.
Consequently, variations for the Query Title block weren't being shown.