libphobos: Remove unused variables in gcc.backtrace.

The core.runtime module always overrides the default parameter value for
constructor calls.  MaxAlignment is not required because a class can be
created on the stack with the `scope' keyword.

libphobos/ChangeLog:

	* libdruntime/core/runtime.d (runModuleUnitTests): Use scope to new
	LibBacktrace on the stack.
	* libdruntime/gcc/backtrace.d (FIRSTFRAME): Remove.
	(LibBacktrace.MaxAlignment): Remove.
	(LibBacktrace.this): Remove default initialization of firstFrame.
	(UnwindBacktrace.this): Likewise.