build: Do not pass -fno-sanitize-memory-param-retval to subr_coverage.c

In the absence of -fsanitize=kernel-memory, the presence of this flag
results in a -Wunused-command-line-argument warning.

MFC after: 1 week

(cherry picked from commit 2ae32f1f8fcc97c3e9c16effa2ee5e41a1a6c364)