Remove unused build configurations from debugserver xcode project

the xcode project file for debugserver exists only to make my
life easier when I'm working only on debugserver and don't need
to build the rest of llvm/lldb.  It had many build configurations
to reflect our old lldb xcode project file, which is long gone.
Removing them to simplify the configurations.

Specifically dropping CustomSwift-Debug, DebugClang, DebugPresubmission,
CustomSwift-Release, BuildAndIntegration.  Keeping Debug & Release.