demux: subtitle: check the FORMAT= value is a number in mpsub

We don't need vlc_strtof_c() since we already have the float value.
We also don't need to check what is after the number (a comment for example).
As long as it's a number it should be FPS. Anything else other than
"FORMAT=TIME" is bogus. So we don't need to get the remaining part of the string.