ArmPkg: remove redundant _ARM_PLATFORM_FLAGS overrides
ArmPkg: remove redundant _ARM_PLATFORM_FLAGS overrides

Our default is already armv7-a, so no need to rewrite the PLATFORM_FLAGS
for that. Also, setting -mfpu=neon is not entirely inappropriate, since
NEON is not mandatory under v7.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Reviewed-by: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>

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