lib: Remove redundant ftrace flag removal

Since architectures can implement ftrace using a variety of mechanisms,
generic code should always use CC_FLAGS_FTRACE rather than assuming that
ftrace is built using -pg.

Since commit:

  2464a609 ("ftrace: do not trace library functions")

... lib/Makefile has removed CC_FLAGS_FTRACE from KBUILD_CFLAGS, so ftrace is
disabled for all files under lib/.

Given that, we shouldn't explicitly remove -pg when building
lib/string.o, as this is redundant and bad form.

Clean things up accordingly.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Coly Li <colyli@suse.de>
Cc: Gary R Hook <gary.hook@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Matthew Wilcox <willy@infradead.org>
Link: https://lkml.kernel.org/r/20190806162539.51918-1-mark.rutland@arm.com