Administration: Always show the filters on media and post list tables.

Previously, the filters were hidden for single posts or attachments, which could only be achieved by editing the URL manually.

The `is_singular()` check was added long before the list tables were introduced, and appears to no longer serve any purpose in the current code.

As a side effect, this resolves an issue where a non-existing attachment ID in the URL would block further search in Media Library.

Props afercia, tomdude, audrasjb, bencroskery, desrosj, SergeyBiryukov.
Fixes #38221.
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