Removed String.startsWith !FABLE_COMPILER condition

[laurentpayot]

Hi, I had an error with String.endsWith when compiling a project using FSharpPlus with Fable. String.endsWith was not surrounded by a !FABLE_COMPILER symbol condition.
A Fable maintainer fixed my issue by making String.endsWith work with Fable, as well as String.startsWith.

This PR simply removes the !FABLE_COMPILER symbol condition around String.startsWith that is not needed anymore.

[wallymathieu]

Do we need to update fable as well?

[laurentpayot]

@wallymathieu Oopsie. Yes, I just bumped Fable to version 4.23.0 so the fix is included.