BaseTools: Remove the redundant __FLEXIBLE_SIZE from PcdValueInit.c

BaseTools: Remove the redundant __FLEXIBLE_SIZE from PcdValueInit.c

For the multiple SKU case, basetools generates multiple redundant
__FLEXIBLE_SIZE statement that is from the same line in dsc file.

This behavior causes the generated PcdValueInit.c file to have huge size,
and the compilation time is very long.

This patch is going to fix this issue.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Reviewed-by: Yuwei Chen <yuwei.chen@intel.com>