Remove LCMS mutex

[deymo]

Describe the bug
We have a mutex in the LCMS color_management which can probably be removed after an update in LCMS. See #23 (comment) for context.

[kleisauke]

I tried to remove that mutex on this branch, but it seems that LCMS still has a thread-safety issue regarding gmtime.
https://github.com/mm2/Little-CMS/blob/807809d7dd9b09c0380b71d5d5ebee3c1439ca99/src/cmsio0.c#L503-L504

TSAN results

Tested branch: main...kleisauke:remove-lcms-mutex

Run with:

$ BUILD_DIR=build_tsan CC=clang-12 CXX=clang++-12 SKIP_TEST=1 ./ci.sh tsan -DJPEGXL_ENABLE_SKCMS=OFF
$ ./build_tsan/lib/tests/passes_test

Commit kleisauke@28c3009:

ThreadSanitizer: reported 8 warnings
(see tsan-lcms2.10.log for more details)

Commit kleisauke@e1a97c9:

ThreadSanitizer: reported 2 warnings
(see tsan-lcms2.12.log for more details)

Attachments:
tsan-lcms2.10.log
tsan-lcms2.12.log

[kleisauke]

A pull request has been made to fix the gmtime issue at mm2/Little-CMS#263.