package/gst1-plugins-base: drop legacy remnant comment

In 76722342 (gst1-plugins-base: bump version to 1.12.0), the unknown
options were removed, but the comment associated to --disable-gio_unix_2_0
was left out.

Drop it now.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>