Remove "Plugin/Sources/protoc-gen-swiftgrpc/templates.swift" from source control.

This file actually gets regenerated by running `make` in the `Plugin` directory, but it is easy to forget to check in changes to it when making changes to the underlying templates. So to avoid inconsistencies, I think it is better to only rely on the user's freshly-generated local copy.

.gitignore

@@ -4,6 +4,7 @@ xcuserdata
 .build
 third_party/**
 Plugin/Packages/**
+Plugin/Sources/protoc-gen-swiftgrpc/templates.swift
 Plugin/protoc-*
 Plugin/swiftgrpc.log
 Plugin/echo.*.swift