mp4: Remove GPAC Windows Unicode compatibility shim

GPAC has native UTF-8 support nowadays.

Also move the compatibility code to input/avs.c since that's the only
remaining code that uses it now.
15 jobs for stable in 9 minutes and 57 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #469620
aarch64 docker
build-debian-aarch64

00:08:22

passed #469619
amd64 docker
build-debian-amd64

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passed #469623
macos
build-macos

00:01:45

passed #469621
amd64 docker
build-win32

00:02:24

passed #469622
amd64 docker
build-win64

00:02:28

 
  Test
passed #469625
aarch64 docker
test-debian-aarch64

00:00:25

passed #469624
amd64 docker
test-debian-amd64

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passed #469628
macos
test-macos

00:00:08

passed #469626
amd64 docker
test-win32

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passed #469627
amd64 docker
test-win64

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  Release
passed #469630
aarch64 docker manual
release-debian-aarch64

00:00:23

passed #469629
amd64 docker manual
release-debian-amd64

00:00:15

passed #469633
macos manual
release-macos

00:00:03

passed #469631
amd64 docker manual
release-win32

00:00:15

passed #469632
amd64 docker manual
release-win64

00:00:16