android: LibraryLoader: replace a comment with an assert

Also rename s/Linkers/Linker/ because we have only one linker. Also
remove a check for >=L.

Bug: None
Change-Id: I78ab7fea2ee5e5d4723747360ff69cb794fc1ce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4064122


Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076985}