configure: Remove mpegvideo_enc dependency from ljpeg encoder

It only existed because some code in mjpegenc_common.c relied on it;
yet said code was actually only used by mjpegenc.c and has been moved
there.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

configure

@@ -2762,7 +2762,7 @@ interplay_video_decoder_select="hpeldsp"
 jpegls_decoder_select="mjpeg_decoder"
 jv_decoder_select="blockdsp"
 lagarith_decoder_select="llviddsp"
-ljpeg_encoder_select="idctdsp jpegtables mpegvideoenc"
+ljpeg_encoder_select="idctdsp jpegtables"
 lscr_decoder_deps="zlib"
 magicyuv_decoder_select="llviddsp"
 magicyuv_encoder_select="llvidencdsp"