INSTALL: remove stale coverage build instruction

Now it is controlled by Kconfig.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

INSTALL

@@ -280,9 +280,6 @@ This option is enabled by default if XSM is enabled and the compiler
 using the environment variable.
 CHECKPOLICY=
 
-Do a build for coverage.
-coverage=y
-
 Use clang instead of GCC.
 clang=y