ShellPkg: remove superfluous TimerLib resolution

ShellPkg: remove superfluous TimerLib resolution

TimerLib had to be resolved in commit 5ab97a64b51c ("ShellPkg/bcfg: Add
Shell Spec 2.2 modification functionality", 2017-03-01) because:

- the BCFG command started making calls to UefiBootManagerLib
  (EfiBootManagerVariableToLoadOption(),
  EfiBootManagerLoadOptionToVariable(), EfiBootManagerFreeLoadOption()),

- and "MdeModulePkg/Library/UefiBootManagerLib/UefiBootManagerLib.inf"
  depended on TimerLib.

Because TimerLib is platform-specific, but "ShellPkg/ShellPkg.dsc" is
meant to produce a UEFI shell binary that is platform-independent (see
"ShellBinPkg/ReadMe.txt"), we resolved TimerLib to
"BaseTimerLibNullTemplate.inf". (TimerLib functionality was never actually
needed on UefiBootManagerLib code paths that were exercised by the shell /
BCFG.)

Thanks to the last patch, UefiBootManagerLib no longer depends on
TimerLib, thus we can drop the TimerLib resolution entirely.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>