Remove custom optional implementation.

[rnlahaye]

It is no longer needed as of #3279.

[pleroy]

[Automated message from GitHub Pull Request Builder] Answer "ok to test" to trigger testing of this PR.

[rnlahaye]

ok to test

[pleroy]

Can we get rid of this one too?

[rnlahaye]

Filesystem requires 10.15, so no.

Clang 12 is willing to compile filesystem while targeting 10.12, but I haven't actually tried running the resulting binary on a pre-Catalina system; I suspect there might be runtime errors in that case.

[eggrobin]

Yeah now that I think of it there is a note to that effect in the script (and a citation of Cicero for good measure):

Principia/install_deps.sh

Lines 33 to 36 in c8bcc2d

	# NOTE(egg): We need Clang 8, and therefore we use Xcode 11. The libc++ provided by
	# Xcode 11 has the <filesystem> header, however its symbols are marked unavailable
	# unless macosx-version-min is at least 10.15 (Catalina). Since we do not want to
	# require that version yet (quousque tandem?), we cannot use the libc++ filesystem.