test: Remove mention of mock/ from test/bpf/Makefile
test: Remove mention of mock/ from test/bpf/Makefile

The mock/ directory was removed with commit df1bc9660746 ("bpf: ported
existing unit tests to new testing framework"), but we still attempt to
clean it with "make -C test/bpf". Let's remove it from the Makefile.

Fixes: df1bc9660746 ("bpf: ported existing unit tests to new testing framework")
Signed-off-by: Quentin Monnet <d6b8e48afb2534b213e391cab43016505747a234@isovalent.com>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium