Previously, the `FileSystem` abstraction featured a `mkdir()` method. In
`NodeJSFileSystem` (the default `FileSystem` implementation used in
actual code), the method behaved similar to Node.js' `fs.mkdirSync()`
(i.e. failing if any parent directory is missing or the directory exists
already). In contrast, `MockFileSystem` (which is the basis or mock
`FileSystem` implementations used in tests) implemented `mkdir()` as an
alias to `ensureDir()`, which behaved more like Node.js'
`fs.mkdirSync()` with the `recursive` option set to `true` (i.e.
creating any missing parent directories and succeeding if the directory
exists already).
This commit fixes this inconsistency by removing the `mkdir()` method,
which was not used anyway and only keeping `ensureDir()` (which is
consistent across our different `FileSystem` implementations).
PR Close#33237
d7dc6cbc04