Revert "Fix configuration of webp libraries"

This reverts commit de4277af00.
It breaks WebP support at least on my system.
There's no reason to require libwebpdecoder library to be
installed, since we don't use the functions from it, at least
not in libwebp 1.2.1.

configure.ac

@@ -2803,8 +2803,7 @@ if test "${with_webp}" != "no"; then
    || test "${HAVE_W32}" = "yes" || test "${HAVE_NS}" = "yes" \
    || test "${HAVE_BE_APP}" = "yes" || test "${HAVE_PGTK}" = "yes"; then
       WEBP_REQUIRED=0.6.0
-      WEBP_MODULE="libwebpdemux >= $WEBP_REQUIRED \
-                   libwebpdecoder >= $WEBP_REQUIRED"
+      WEBP_MODULE="libwebpdemux >= $WEBP_REQUIRED"
 
       EMACS_CHECK_MODULES([WEBP], [$WEBP_MODULE])
       AC_SUBST([WEBP_CFLAGS])