Simplify assembly load on CoreCLR

[davidfowl]

Just use the default load context for razor views. We get no benefit from having a custom load context. Now that we can load things directly into the default load context, we don't need it.

/cc @pranavkm

[Eilon]

@davidfowl needed for RC2?

[davidfowl]

Nah, can wait until RTM. Though the change is minimal.

[Eilon]

Ok will do in RTM.