fix: JPEG detection fails on files with trailing data (Motion Photos)

[tetsugakusha256]

detect_format() in magic.lua has a check that requires FFD9 (JPEG EOI) to
be the literal last 2 bytes of the file. That breaks on any JPEG that has
data appended after the actual image — Google/Samsung Motion Photos being
the common case (JPEG + embedded MP4 + trailer footer).

For these files the real EOI is somewhere in the middle, so
has_jpeg_end_signature() returns false and detect_format() returns nil for
the whole file, even though it's a perfectly valid JPEG. ImageMagick has
no problem with these files (tested manually with identify/convert), it
just stops reading at EOI like it's supposed to.

I don't think this check is worth keeping — no other format in the table
gets this kind of extra validation, and detect_format() has no fallback
to the identify-based path used elsewhere in the plugin, so this was a
silent permanent failure rather than just a perf hit.

Removed the EOI check and the now-unused has_jpeg_end_signature function.

Tested:

• Motion Photo JPEGs now detected/rendered correctly
• truncated JPEGs still fail sensibly downstream - partial render if
  there's enough data, clean identify error ("insufficient image data")
  if not