Remove declaration of constexpr member kDynamicSize in MemRefType

This member is already publicly declared on the base class. The
redundant declaration is mangled differently though and in some
unoptimized build it requires a definition to also exist. However we
have a definition for the base ShapedType class, removing the
declaration here will redirect every use to the base class member
instead.

Differential Revision: https://reviews.llvm.org/D84615