remove conflicting default call in tables.getOrDefault (#24265)

fixes #23587

As explained in the issue, `getOrDefault` has a parameter named
`default` that can be a proc after generic instantiation. But the
parameter having a proc type [overrides all other
overloads](f73e03b132/compiler/semexprs.nim (L1203))
including the magic `system.default` overload and causes a compile error
if the proc doesn't match the normal use of `default`. To fix this, the
`result = default(B)` initializer call is removed because it's not
needed, `result` is always set in `getOrDefaultImpl` when a default
value is provided.

This is still a suspicious behavior of the compiler but `tables` working
has a higher priority.

(cherry picked from commit 67ea754b7f)

compiler/docgen.nim

@@ -115,7 +115,7 @@ proc add(dest: var ItemPre, str: string) = dest.add ItemFragment(isRst: false, s
 
 proc addRstFileIndex(d: PDoc, fileIndex: lineinfos.FileIndex): rstast.FileIndex =
   let invalid = rstast.FileIndex(-1)
-  result = d.nimToRstFid.getOrDefault(fileIndex, default = invalid)
+  result = d.nimToRstFid.getOrDefault(fileIndex, invalid)
   if result == invalid:
     let fname = toFullPath(d.conf, fileIndex)
     result = addFilename(d.sharedState, fname)