Revert "gst-ffmpeg: fix build for xtensa"

This reverts commit 7e558aa0.
Now that -mtext-section-literals is specified in the xtensa ABI this fix
is no longer needed.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>