Remove an incorrect CookieParser test. (dotnet/corefx#26927)

Remove an incorrect CookieParser test. (dotnet/corefx#26927)

The test CookieParserGet_SetCookieHeaderValue_Success was written with an incorrect understanding of the desired behavior of CookieParser.Get().

The test was disabled when it was merged, and is fundamentally incorrect enough that it doesn't make sense to rewrite it. The relevant code is already covered by the regular cookie parsing tests.

Fixes: dotnet/corefx#22925

Commit migrated from https://github.com/dotnet/corefx/commit/ff003980926913d799850233ea59b1ebee3ed837