gccrs: Remove obsolete classes and functions.

Trait functions now use AST::Function class, so classes
AST::TraitItemFunc, AST::TraitItemMethod, AST::TraitFunctionDecl,
AST::TraitMethodDecl and their related functions can be removed.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit):
	Remove obsolete classes and functions.
	* ast/rust-ast-collector.h:
	Likewise.
	* ast/rust-ast-full-decls.h (class TraitFunctionDecl):
	Likewise.
	(class TraitItemFunc):
	Likewise.
	(class TraitMethodDecl):
	Likewise.
	(class TraitItemMethod):
	Likewise.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit):
	Likewise.
	* ast/rust-ast-visitor.h:
	Likewise.
	* ast/rust-ast.cc (TraitItemFunc::TraitItemFunc):
	Likewise.
	(TraitItemFunc::operator=):
	Likewise.
	(TraitItemFunc::as_string):
	Likewise.
	(TraitFunctionDecl::as_string):
	Likewise.
	(TraitItemMethod::TraitItemMethod):
	Likewise.
	(TraitItemMethod::operator=):
	Likewise.
	(TraitItemMethod::as_string):
	Likewise.
	(TraitMethodDecl::as_string):
	Likewise.
	(TraitItemFunc::accept_vis):
	Likewise.
	(TraitItemMethod::accept_vis):
	Likewise.
	* ast/rust-item.h (class TraitFunctionDecl):
	Likewise.
	(class TraitItemFunc):
	Likewise.
	(class TraitMethodDecl):
	Likewise.
	(class TraitItemMethod):
	Likewise.
	* checks/errors/rust-ast-validation.cc (ASTValidation::visit):
	Likewise.
	* checks/errors/rust-ast-validation.h:
	Likewise.
	* checks/errors/rust-feature-gate.h:
	Likewise.
	* expand/rust-cfg-strip.cc (CfgStrip::maybe_strip_trait_function_decl):
	Likewise.
	(CfgStrip::maybe_strip_trait_method_decl):
	Likewise.
	(CfgStrip::visit):
	Likewise.
	* expand/rust-cfg-strip.h:
	Likewise.
	* expand/rust-derive.h:
	Likewise.
	* expand/rust-expand-visitor.cc (ExpandVisitor::expand_trait_function_decl):
	Likewise.
	(ExpandVisitor::expand_trait_method_decl):
	Likewise.
	(ExpandVisitor::visit):
	Likewise.
	* expand/rust-expand-visitor.h:
	Likewise.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::visit):
	Likewise.
	* hir/rust-ast-lower-base.h:
	Likewise.
	* hir/rust-ast-lower-implitem.cc (ASTLowerTraitItem::visit):
	Likewise.
	* hir/rust-ast-lower-implitem.h:
	Likewise.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit):
	Likewise.
	* resolve/rust-ast-resolve-base.h:
	Likewise.
	* resolve/rust-ast-resolve-implitem.h (visit):
	Likewise.
	* resolve/rust-ast-resolve-item.cc (ResolveTraitItems::visit):
	Likewise.
	* resolve/rust-ast-resolve-item.h:
	Likewise.
	* resolve/rust-default-resolver.cc (DefaultResolver::visit):
	Likewise.
	* resolve/rust-default-resolver.h:
	Likewise.
	* resolve/rust-toplevel-name-resolver-2.0.cc (TopLevel::visit):
	Likewise.
	* resolve/rust-toplevel-name-resolver-2.0.h:
	Likewise.
	* util/rust-attributes.cc (AttributeChecker::visit):
	Likewise.
	* util/rust-attributes.h:
	Likewise.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>