build: deb: enable apparmor by default & remove deb-apparmor

The official .deb package is always built with apparmor support, so use
`--enable-apparmor` in mkdeb.sh and remove the "deb-apparmor" target in
order to reduce redundancy.

Note that custom configure options may be specified by calling
./mkdeb.sh directly.

For example, to build the .deb package without apparmor support, instead
of running `make deb`, the following commands can be used:

    make dist
    ./mkdeb.sh --disable-apparmor

Also, change the `build_apparmor` GitLab CI job into
`build_no_apparmor`, which is intended to check that building without
apparmor still works.

Note: This commit makes the resulting .deb package not have an
"-apparmor" suffix (see `EXTRA_VERSION` in mkdeb.sh), to avoid
redundancy (as having apparmor support becomes the default).

Misc: This is a follow-up to #5654.

Relates to #5154 #5176 #5547.

mkdeb.sh

@@ -25,7 +25,7 @@ echo "*****************************************"
 tar -xJvf "$CODE_ARCHIVE"
 #mkdir -p "$INSTALL_DIR"
 cd "$CODE_DIR"
-./configure --prefix=/usr "$@"
+./configure --prefix=/usr --enable-apparmor "$@"
 make -j2
 mkdir debian
 DESTDIR=debian make install-strip