[ELF] Remove unused setSymbolAndType after #69295. NFC

One use of setSymbolAndType (related to https://reviews.llvm.org/D53864
"Do not crash when -r output uses linker script with `/DISCARD/`")
is no longer needed after commit 1981b1b
demotes symbols in discarded sections to Undefined.

lld/ELF/InputSection.cpp

@@ -435,10 +435,7 @@ void InputSection::copyRelocations(uint8_t *buf,
         continue;
       }
       SectionBase *section = d->section;
-      if (!section->isLive()) {
-        p->setSymbolAndType(0, 0, false);
-        continue;
-      }
+      assert(section->isLive());
 
       int64_t addend = rel.addend;
       const uint8_t *bufLoc = sec->content().begin() + rel.offset;