gcc-plugins: remove SANCOV gcc plugin

With the minimum gcc version raised to 8.1, all supported compilers
now understand the -fsanitize-coverage=trace-pc option, and there
is no longer a need for the separate compiler plugin.

Since only gcc-5 was able to use the plugin for several year now,
it was already likely unused.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>