arch/Config.in.powerpc: remove unused gcc target abi options for powerpc

gcc target abi options for powerpc were added by [1] and renamed by [2]
to BR2_PPC_ABI_* but never used. Since always BR2_GCC_TARGET_ABI is empty
when using a powerpc toolchain.

Buildroot currently support SPE and Classic target ABI, nothing seems
to require a specific gcc target abi option.

This patch is a cleanup like commit [3].

[1] 7d8a59b4
[2] 98175bd4
[3] fd08153b

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Cyril Bur <cyrilbur@gmail.com>
Cc: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>