BaseTools/tools_def.template: remove GCC48_IA32_X64_DLINK_COMMON dead-end

BaseTools/tools_def.template: remove GCC48_IA32_X64_DLINK_COMMON dead-end

DLINK_COMMON definitions are not consumed by "build_rule.template";
instead, DLINK_COMMON definitions (internal to "tools_def.template") were
invented for sharing options between ASLDLINK_FLAGS and DLINK_FLAGS.

However, this intent doesn't actually apply to
GCC48_IA32_X64_DLINK_COMMON: it is never consumed. Furthermore, the
GCC45..GCC47 instances of IA32_X64_DLINK_COMMON too lead up to
GCC48_IA32_X64_DLINK_COMMON only -- they form a dead-end. Remove them
altogether, in order to simplify the subsequent patches.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1377
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>