dist/debian: drop unused Makefile variable

this change was previously reverted by
cbc005c6f5 . it turns out this change
was but the offending change. so let's resurrect it.

`job` was introduced back in 782ebcece4,
so we could consume the option specified in DEB_BUILD_OPTIONS
environmental variable. but now that we always repackage
the artifacts prebuilt in the relocatable package. we don't build
them anymore when packaging debian packages. see
9388f3d626 . and `job` is not
passed to `ninja` anymore.

so, in this change, `job` is removed from debian/rules as well, as
it is not used.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes #12924

dist/debian/debian/rules

@@ -3,7 +3,6 @@
 include /usr/share/dpkg/pkg-info.mk
 
 export PYBUILD_DISABLE=1
-jobs := $(shell echo $$DEB_BUILD_OPTIONS | sed -r "s/.*parallel=([0-9]+).*/-j\1/")
 ifneq ($(findstring housekeeping, $(DEB_BUILD_OPTIONS)),)
     install_arg := --housekeeping
 else