Fix/strip path fields from component schemas

[paultyng]

When using google.api.http with path parameters, path-bound fields are correctly excluded from inline request bodies (fixed in #234), but they still appear in the corresponding components/schemas entries. This adds an opt-in strip-path-params-from-schemas option to remove them there as well.
Related: #27

Example

Given this proto:

message GetThingRequest {
  string path = 1 [(google.api.field_behavior) = REQUIRED];
}
service ThingService {
  rpc GetThing(GetThingRequest) returns (Thing) {
    option (google.api.http) = {
      get: "/v1/{path=things/*}"
    };
  }
}

Before — path appears as both a URL parameter and in the component schema:

paths:
  /v1/things/{thing}:
    get:
      parameters:
        - name: thing
          in: path
          required: true
components:
  schemas:
    GetThingRequest:
      type: object
      properties:
        path:          # redundant — already a path param
          type: string
      required:
        - path

After (with strip-path-params-from-schemas):

components:
  schemas:
    GetThingRequest:
      type: object

This matches how Google Cloud APIs handle the same situation in their Discovery Documents — path-bound fields are not repeated in request schemas.

I made this opt-in so as not to impact existing generation expectations, but I think it is perhaps a bug, unsure?