package/bc: remove default value of _SOURCE var

check-package output:
package/bc/bc.mk:8: remove default value of _SOURCE variable

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/295683825

Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/bc/bc.mk

@@ -5,7 +5,6 @@
 ################################################################################
 
 BC_VERSION = 1.07.1
-BC_SOURCE = bc-$(BC_VERSION).tar.gz
 BC_SITE = http://ftp.gnu.org/gnu/bc
 BC_DEPENDENCIES = host-flex
 BC_LICENSE = GPL-2.0+, LGPL-2.1+