Windows: Remove unnecessary codepath from modname.

File names starting with periods are perfectly acceptable on Windows
file systems. The only place where this is not acceptable is on
MS-DOS FAT file systems which only support 8.3 file names.

See here:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx

Since Neovim does not support MS-DOS or 8.3 file names (#605)
we can drop this codepath.

It was not compiling anyways since we do not define WIN3264.

src/nvim/fileio.c

@@ -4331,8 +4331,6 @@ void shorten_fnames(int force)
 /// @return [allocated] - A new filename, made up from:
 ///                       * fname + ext, if fname not NULL.
 ///                       * current dir + ext, if fname is NULL.
-///                         On Windows, and if ext starts with ".", a "_" is
-///                         preprended to ext (for filename to be valid).
 ///                       Result is guaranteed to:
 ///                       * be ended by <ext>.
 ///                       * have a basename with at most BASENAMELEN chars:
@@ -4386,15 +4384,6 @@ char *modname(const char *fname, const char *ext, bool prepend_dot)
   char *s;
   s = ptr + strlen(ptr);
 
-#if defined(WIN3264)
-  // If there is no file name, and the extension starts with '.', put a
-  // '_' before the dot, because just ".ext" may be invalid if it's on a
-  // FAT partition, and on HPFS it doesn't matter.
-  else if ((fname == NULL || *fname == NUL) && *ext == '.') {
-    *s++ = '_';
-  }
-#endif
-
   // Append the extension.
   // ext can start with '.' and cannot exceed 3 more characters.
   strcpy(s, ext);