Update Merge request file tree placeholder text to align with Source Code file tree text

Context

Feedback on the placeholder text was discussed for the Source Code file tree browser in #558792 (comment 2665841502).

Design recommendation

Update the Merge request > Changes tab file tree placeholder text to align with the the Source Code file tree, so both are consistent.

• e.g. should be removed due to localization issues #558792 (comment 2682688540)
• (F) should be removed because it is a global filter/search shortcut, available on every page where a filter or search input exists !199595 (comment 2657826689)

Current	Suggested placeholder text
Search (e.g. *.vue) (F)	Filter files (*.vue, *.rb...)