Remove -lpthread from --extra-libs

-lpthread was previously required for building with libaom support.
Currently it seems that upstream aom git master has handled this
missing library to its linkage, and this option is not needed here
anymore.

References
----------
https://aur.archlinux.org/cgit/aur.git/commit/?h=ffmpeg-full-git&id=a57553ab08c7af23735921498c4393e320b0553b

.SRCINFO

@@ -1,7 +1,7 @@
 pkgbase = ffmpeg-full
 	pkgdesc = Complete solution to record, convert and stream audio and video (all possible features including nvenc, qsv and libfdk-aac)
 	pkgver = 4.0
-	pkgrel = 5
+	pkgrel = 6
 	url = http://www.ffmpeg.org/
 	arch = i686
 	arch = x86_64

PKGBUILD

@@ -4,7 +4,7 @@
 pkgname=ffmpeg-full
 _srcname=ffmpeg
 pkgver=4.0
-pkgrel=5
+pkgrel=6
 pkgdesc='Complete solution to record, convert and stream audio and video (all possible features including nvenc, qsv and libfdk-aac)'
 arch=('i686' 'x86_64')
 url='http://www.ffmpeg.org/'
@@ -96,7 +96,6 @@ build() {
         --prefix='/usr' \
         --extra-cflags="$_cflags" \
         --extra-ldflags="$_ldflags" \
-        --extra-libs='-lpthread' \
         \
         --disable-rpath \
         --enable-gpl \