tests/string: Drop simple/stupid/builtin tests

In most cases the simple/stupid/builtin functions were in there to
benchmark optimized implementations against.  Only in some cases the
functions are used to check expected results.

Remove these tests from IMPL() and only keep them in wherever they're
used for a specific purpose, e.g. to generate expected results.

This improves timing of `make subdirs=string` by over a minute and a
half (over 15%) on a Whiskey Lake laptop.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Noah Goldstein <libc-alpha@sourceware.org>