[ELF][NFCI] small cleanup to OutputSections.h

OutputSections.h used to close the lld::elf namespace only to
immediately open it again. This change merges both parts into
one.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D94538

lld/ELF/OutputSections.h

@@ -135,12 +135,6 @@ struct Out {
   static OutputSection *finiArray;
 };
 
-} // namespace elf
-} // namespace lld
-
-namespace lld {
-namespace elf {
-
 uint64_t getHeaderSize();
 
 extern std::vector<OutputSection *> outputSections;