[cmake] fix link issues of unit tests (#5309)

I think openthread-ncp-ftd is added to unit tests' link libraries by
mistake. It's not required and as it depends on OT_PLATFORM, it would
cause openthread-simulation linked in unit test executables, which
would lead to a redefinition as some methods are defined in
test-platform.cpp.

tests/unit/CMakeLists.txt

@@ -33,7 +33,6 @@ set(COMMON_SOURCES
 
 set(COMMON_LIBS
     openthread-ftd
-    openthread-ncp-ftd
     ot-config
     ${OT_MBEDTLS}
     util