Revert "Expose Marshall.LoadLibrary API" (dotnet/corefx#34322)

Revert "Expose Marshall.LoadLibrary API" (dotnet/corefx#34322)

In API review: https://github.com/dotnet/corefx/issues/32015,

The LoadLibrary APIs were originally Approved to live in
System.Runtime.InteropServices.Marshal class.
https://github.com/dotnet/corefx/issues/32015#issuecomment-428775858

However, recently the decision was changed such that the APIs are in a new NativeLibrary class.
https://github.com/dotnet/corefx/issues/32015#issuecomment-448324606

Therefore, undoing the changes to Marshal API.
I'll submit another PR for System.Runtime.InteropServices.NativeLibrary
once the corresponding change in CoreCLR is checked in.

This reverts commit dotnet/corefx@859351c2f65e73e94c9c40983e53b33c5b45014a.

Commit migrated from https://github.com/dotnet/corefx/commit/6d59e6ab54bb313ea5eb6a809dc64a7cebe8ee64