sanitycheck: native: Removed --coverage from EXTRA_LDFLAGS

Removed redundant linker flag --coverage from native posix
EXTRA_LDFLAGS.

The correct linker flags will be set by defining the Kconfig flag
CONFIG_COVERAGE, either by sanitcheck it self or via the test/sample
prj.conf.

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>