chore(build): remove grouped-target compatibility variable

Our prerequisites specify GNU Make 4.3 is required to build TF-A; the
latest Ubuntu release an older version appears in was 20.04, which is no
longer generally supported as of May 2025.

Change-Id: Ib3923895c88c88867260212afab4aa8094e224ca
Signed-off-by: Chris Kay <chris.kay@arm.com>