ShellPkg/ShellLib: Remove unused macros

MAX_FILE_NAME_LEN and FIND_XXXXX_FILE_BUFFER_SIZE are not used
by ShellLib so remove them.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>