package/ejabberd: remove useless _SOURCE variable

In commit fd29a61f019146b52e0056c7ae8a2bb9bd7c086c ("package/ejabberd:
bump version to 23.04"), the EJJABERD_SOURCE variable was changed to
"ejabberd-$(EJABBERD_VERSION).tar.gz", which is the default value for
the _SOURCE variable, causing check-package to complain. Fix that by
dropping the useless variable definition.

Fixes:

  package/ejabberd/ejabberd.mk:8: remove default value of _SOURCE variable (http://nightly.buildroot.org/#generic-package-reference)

In:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/4718833134

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/ejabberd/ejabberd.mk

@@ -5,7 +5,6 @@
 ################################################################################
 
 EJABBERD_VERSION = 23.04
-EJABBERD_SOURCE = ejabberd-$(EJABBERD_VERSION).tar.gz
 EJABBERD_SITE = https://static.process-one.net/ejabberd/downloads/$(EJABBERD_VERSION)
 EJABBERD_LICENSE = GPL-2.0+ with OpenSSL exception
 EJABBERD_LICENSE_FILES = COPYING