Remove gas-preprocessor usage on iOS

gas-preprocessor has been reintroduced in [1] so as to re-enable the iOS armv7 CI in [2].

According to this [3] thread, this shouldn't be needed anymore.

Make sure this is not needed and remove usage.

[1] 6ec0aadc apple: build: build gas-preprocessor for armv7

[2] 972a70d6 gitlab-ci: add armv7 iOS CI

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