Remove tests-fwu.mk from tests-standard.mk

The FWU test components are being built as part of the
tests-standard.mk configuration but tests-standard does not
actually run any FWU tests (see tests-standard.xml). These FWU
test components do not need to be built.

Change-Id: I11a9012fcf79f4066947b0b2801c64d053f38d42
Signed-off-by: Paul Beesley <paul.beesley@arm.com>