Use libm_alias_double for dbl-64 atan, tan.

This patch makes the dbl-64 atan and tan implementations use
libm_alias_double, removing the corresponding ldbl-opt wrappers.

Tested for x86_64, and with build-many-glibcs.py.  Installed stripped
shared libraries are unchanged on non-ldbl-opt platforms.  For
ldbl-opt configurations, the patch has the effect of causing
compat_symbol to define atanl and tanl in terms of __atan and __tan
instead of in terms of atan and tan, which is enough to change the
installed stripped libm.so.

	* sysdeps/ieee754/dbl-64/s_atan.c: Include <libm-alias-double.h>.
	(atan): Define using libm_alias_double.
	* sysdeps/ieee754/dbl-64/s_tan.c: Include <libm-alias-double.h>.
	(tan): Define using libm_alias_double.
	* sysdeps/ieee754/ldbl-opt/s_atan.c: Remove file.
	* sysdeps/ieee754/ldbl-opt/s_tan.c: Likewise.