don't raise ProxyError in VerifiedHTTPSConnection

Reverts a part of 51d485a (introduced in #221)

Before this path SocketErrors were always wrapped in ProxyErrors.
This should only happen if the connection is to a proxy.
Just remove the check as it will happen anyway in HTTPConnectionPool.url_open
Merge pull request #235 from t-8ch/incorrect_proxyerror

don't raise ProxyError in VerifiedHTTPSConnection