TextureCommand::GenerateMipmap can be moved into gl::Command allowing us
to remove TextureCommand.

Bug: angleproject:4753
Test: CQ
Change-Id: Idc546df519e199ffd3a8b8e03b9868cd9152e9ef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2338823
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>