Revert "backend-drm: add HDR_OUTPUT_METADATA definitions"

This reverts commit 6914064066.

This is a follow-up change of b623fd2a ("drm-backend: stop parsing IN_FORMATS
blobs, use libdrm instead"). Weston now has a hard-requirement on libdrm
2.4.108, clean up remaining and unnecessary conditional code. Change 69140640
("backend-drm: add HDR_OUTPUT_METADATA definitions") is no longer needed
and stop including libdrm-updates.h from kms-color.c.

Signed-off-by: Luigi Santivetti <luigi.santivetti@imgtec.com>

doc/sphinx/doxygen.ini.in

@@ -888,7 +888,7 @@ EXCLUDE_SYMLINKS       = NO
 # Note that the wildcards are matched against the file with absolute path, so to
 # exclude all test directories for example use the pattern */test/*
 
-EXCLUDE_PATTERNS       = */backend-drm/libdrm-updates.h
+EXCLUDE_PATTERNS       =
 
 # The EXCLUDE_SYMBOLS tag can be used to specify one or more symbol names
 # (namespaces, classes, functions, etc.) that should be excluded from the