GDB_TARGET_HAS_SHARED_LIBS #ifdef and update comment to
indicate that the condition it was testing is always true.
* config/alpha/nm-linux.h (GDB_TARGET_HAS_SHARED_LIBS): Remove.
* config/alpha/nm-nbsd.h (GDB_TARGET_HAS_SHARED_LIBS): Ditto.
* config/alpha/nm-osf.h (GDB_TARGET_HAS_SHARED_LIBS): Ditto.