Previously in the `//`-compiletest-directive times, this was implemented
as a special `no-*` directive parsing. In the migration from `//` ->
`//@`, the `// no-remap-src-base` directive was lost, most likely
because it had no effect -- the default is not remapping `src-base`.
So remove occurrences of `no-remap-src-base`, as these are not valid
directives.
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