Remove FormulaInstaller.prevent_build_flags

[Bo98]

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Responsibility of args should lie with the command itself. The args to check for here varied depending on whether it was an install, reinstall or upgrade.

If we want some sort of shared logic, I'd rather it was done in a more generic way - such as having some way to tag which options are "build flags" and iterating over those.

Fixes Homebrew/discussions#1104.

[BrewTestBot]

Review period will end on 2021-03-23 at 16:15:39 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, thanks!