Rollup merge of #139528 - Zalathar:no-logfile, r=jieyouxu

compiletest: Remove the `--logfile` flag

This flag is deprecated in libtest (#134283), and there's no evidence in-tree of this flag actually being passed to compiletest.

For detailed information about test results, bootstrap parses JSON output from compiletest instead (#108659).

As part of my experimental work on removing the libtest dependency from compiletest, it's useful to be able to disconnect libtest functionality that isn't needed.

Author: matthiaskrgr