Remove unescaped spans support from RuleBuilder

Now that rsp files use an explicit path instead of $out.rsp the
unescaped spans and NinjaEscapedCommands() support can be removed
from RuleBuilder.

Bug: 182612695
Test: rule_builder_test.go
Change-Id: I6705151ff0528c59aa1af56ee52c294da15a7a66