[PECOFF] /safeseh:no on x64 is not an error

I made LLD to report an error if /safeseh:no option is given on x64,
but it turned out MSVC link.exe doesn't report error on it.
Removing the check.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218077 91177308-0d34-0410-b5e6-96231b3b80d8