[lldb] Remove Windows XFAIL for TestDollarInVariable.py

Since 5d66f9fd this test has
been upassing on Linaro's Windows on Arm lldb bot:
https://lab.llvm.org/buildbot/#/builders/219/builds/4320

I can't explain exactly how that happened, but I do see a bunch
of QEnvironment packets going by in that test. It is very likely
that the order would have been different on Windows.

Indeed, when it was xfailed back in df9051e7
the reason was not known either.