Revert "Fix edge case test failure in ext/POSIX/t/mb.t"

This reverts commit 69b89a0.

This commit generated two test failures on Linux when perl was built
with config_args like these:

'-des -Dusedevel -Dcc=clang -Accflags=-Werror=declaration-after-statement -g -fno-omit-frame-pointer -fsanitize=address  -fno-common -fsanitize-blacklist=/home/jkeenan/gitwork/perl/asan_ignore -Aldflags=-fsanitize=address'

See: https://rt.perl.org/Ticket/Display.html?id=134182#txn-1639258

Reverting from blead; will create branch for testing.

Signed-off-by: James E Keenan <jkeenan@cpan.org>

ext/POSIX/t/mb.t

@@ -34,13 +34,9 @@ SKIP: {
     my $utf8_locale = find_utf8_ctype_locale();
     skip("no utf8 locale available", 3) unless $utf8_locale;
 
-    # Here we need to influence LC_CTYPE, but it's not enough to just
-    # set this because LC_ALL could override it. It's also not enough
-    # to delete LC_ALL because it could be used to override other
-    # variables such as LANG in the underlying test environment.
-    # Continue to set LC_CTYPE just in case...
     local $ENV{LC_CTYPE} = $utf8_locale;
-    local $ENV{LC_ALL} = $utf8_locale;
+    local $ENV{LC_ALL};
+    delete $ENV{LC_ALL};
 
     fresh_perl_like(
         'use POSIX; print &POSIX::MB_CUR_MAX',