Rollup merge of #149911 - Zalathar:no-color, r=jieyouxu

bootstrap: Don't pass an unused `--color` to compiletest

- Follow-up to rust-lang/rust#149850

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This flag was an artifact of compiletest's old libtest-based test executor, and currently doesn't influence compiletest's output at all.

A follow-up commit also inlines `force_coloring_in_ci` into its only remaining caller, and updates its comment.

Author: jhpratt