Remove `current_blog_` cache invalidation from `clean_blog_cache()`

[flixos90]

clean_blog_cache() currently invalidates keys current_blog_{$blog->domain} and current_blog_{$blog->domain}_{$blog->path} of the site-options group. As far as I can see those keys are not used anywhere in Core (and have not been used for quite a while), so we should be able to remove them from that function.

Related: #40201 (more elaborate improvements of clean_blog_cache())

[spacedmonkey]

I checked and these cache keys haven't been used in core for sometime. It should be safe to remove them.

[thamaraiselvam]

I am also confirming that these keys are not used anywhere in the core. better to remove them.

[DrewAPicture]

Assigning ownership to mark the good-first-bug as "claimed".

[flixos90]

@jeremyfelt @johnjamesjacoby I am not sure, but I think I remember there were some concerns about removing these?

[SergeyBiryukov]

Some history on these cache keys:

• Introduced in ​mu:1789.
• Added to WP in [12602] / [12603].
• Moved to wp-includes/blogs.php in [13125].
• Moved to clean_blog_cache() in [22092].
• Unused since [27359].

[SergeyBiryukov]

In 46990:

Networks and Sites: Remove invalidation of unused cache keys in clean_blog_cache().

Props spacedmonkey, flixos90, thamaraiselvam.
Fixes #40363.

[SergeyBiryukov]

In 46991:

Tests: Remove unused cache keys from data_get_site_caches().

Follow-up to [46990].

See #40363.