Remove InitForm::index (#6817)

This ensures that equal forms always have equal representations (because
the index depends on how the form is used, not on the value of the form
itself).

As a byproduct, also remove `NextCallParamIndex`.

---------

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Co-authored-by: Richard Smith <richard@metafoo.co.uk>
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toolchain/check/pattern.cpp

@@ -165,9 +165,6 @@ auto AddParamPattern(Context& context, SemIR::LocId loc_id,
 
   const auto& param_pattern_kind =
       is_ref ? SemIR::RefParamPattern::Kind : SemIR::ValueParamPattern::Kind;
-  // Allocate a dummy index to preserve index of subsequent `InitForm`s.
-  // TODO: Remove this once we remove `InitForm::index`.
-  context.full_pattern_stack().NextCallParamIndex();
   pattern_id = AddPatternInst(
       context,
       SemIR::LocIdAndInst::UncheckedLoc(