access: rtp: remove explicit _DEPENDENCIES

The _DEPENDENCIES variable is automatically filled when adding internal
convenience libraries to _LIBADD and is not supposed to be overriden.

With LIBADD only, the dependency leads to the following makefile output:

    am__DEPENDENCIES_25 = libvlc_srtp.la \
            $(am__DEPENDENCIES_1)
    librtp_plugin_la_DEPENDENCIES = libvlc_rtp.la $(am__DEPENDENCIES_1) \
            $(am__DEPENDENCIES_25)

modules/access/rtp/Makefile.am

@@ -29,7 +29,6 @@ noinst_LTLIBRARIES += libvlc_srtp.la
 librtp_plugin_la_CPPFLAGS += -DHAVE_SRTP
 librtp_plugin_la_CFLAGS += $(GCRYPT_CFLAGS)
 librtp_plugin_la_LIBADD += libvlc_srtp.la $(GCRYPT_LIBS)
-librtp_plugin_la_DEPENDENCIES = libvlc_srtp.la
 endif
 
 noinst_HEADERS += access/rtp/fmtp.h