Revert a change that shouldn't have been backported to 49.x

Revert a change that shouldn't have been backported to 49.x

Upstream suggested this as a possible trigger for regressions in
modesetting during first login on some systems (the regression seems to
be specific to AMD GPUs, but not all AMD GPUs are affected) and it
seems that it was developed for v50, under the assumption that gdm3
changes from v50 would be present.

For whatever reason (possibly timing-dependent), this was reported
in Debian 13 but not in testing/unstable.

Closes: #1125941, #1125273, #1125275, #1126174