cleanup(generator): remove unused var

[dbolduc]

Part of the work for #14510

This was being set to a nonsense value, so stop setting it and checking for it.

This is how we use the method_request_params variable:

google-cloud-cpp/generator/internal/metadata_decorator_generator.cc

Lines 45 to 48 in a86e887

	if (HasHttpRoutingHeader(method)) {
	return absl::StrCat(" SetMetadata(", context, ", ", options,
	", absl::StrCat($method_request_params$));");
	}

Note that HasHttpRoutingHeader() == false when something is a HttpSimpleInfo.

google-cloud-cpp/generator/internal/http_option_utils.cc

Lines 430 to 433 in a86e887

	bool HasHttpRoutingHeader(MethodDescriptor const& method) {
	auto result = ParseHttpExtension(method);
	return absl::holds_alternative<HttpExtensionInfo>(result);
	}

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.59%. Comparing base (a86e887) to head (9e1b409).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14517   +/-   ##
=======================================
  Coverage   93.58%   93.59%           
=======================================
  Files        2316     2316           
  Lines      207108   207105    -3     
=======================================
+ Hits       193826   193833    +7     
+ Misses      13282    13272   -10     

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