abg-ir.cc: Remove unused re_canonicalize function.

abg-ir.cc: Remove unused re_canonicalize function.

The function re_canonicalize was added in commit:

  286cadf8 Bug 24430 - Fold away const for array types

but later rendered obsolete by commit:

  5d6af8d5 Delay canonicalization for array and qualified types

This commit removes the function and its associated declarations.

	* include/abg-fwd.h (re_canonicalize): Remove declaration of
	obsolete function.
	* include/abg-ir.h (class {decl_base, type_base}): Remove
	re_canonicalize friend declarations from these classes.
	* src/abg-ir.cc (re_canonicalize): Remove obsolete function.

Signed-off-by: Giuliano Procida <gprocida@google.com>