Improve std.regex build times by removing a formattedWrite call

[rikkimax]

I don't know how much we can actually do to speed it up, but this is at least still a win at about ~100ms. That shouldn't change anything other than build times.

Tracking issue (does not close): https://issues.dlang.org/show_bug.cgi?id=23737

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Testing this PR locally

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[rikkimax]

And now we learn why that formattedWrite was there.

std/regex/internal/parser.d(1015): Error: cannot append type wstring to type string
std/regex/internal/parser.d(1017): Error: cannot append type wstring to type string