tests: clean up bionic-unit-tests-static linking

Adding libc++_static explicitly is not needed when it is specified
via stl:, and was the real reason we needed --allow-multiple-definition.

Bug: N/A
Test: ran static tests (32- and 64-bit)
Change-Id: I254a4b8c4f7955a17c970d31b427b05c1a2e0acf
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>