package/gcc: remove powerpc conditional patching logic

Back in commit [1], a patch fixing an issue a PowerPC issue in gcc was
added in gcc 4.3.3. It was present until gcc 4.9, which itself was
removed in [2]. The patch was dropped starting gcc 5.1 [3] but it's
know to be useful for gcc 4.7.3 [4]. However, even though we no longer
support building any of those older gcc versions, the conditional
patching logic in gcc.mk is still there.

We used to have a patch directory (package/gcc/$(GCC_VERSION)) for
every gcc version available in Buildroot, the apply-patches.sh script
doesn't error out even if
1000-powerpc-link-with-math-lib.patch.conditional is missing.

But with gcc 10, we don't need (for the moment) to apply any patch, so
the patch directory doesn't exist. apply-patches.sh breaks the build
since the patch directory is missing:

  Aborting.  'package/gcc/10.1.0' is not a directory.

Since we removed gcc 4.9 last year [2], we can safely remove this code.

Tested using qemu_ppc_virtex_ml507_defconfig.

[1] bb1f42e4
[2] baf17750
[3] 4deb2d93
[4] 197006a4

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>