options: move "-o threads=" help text to libfuse/

Parsing of and acting upon "-o threads" option is done in the bundled `libfuse/`,
so move the corresponding help text there too.
This make help text consistent with behavior
when building against system libfuse,
which doesn't support this option.

src/option_parser.cpp

@@ -299,9 +299,6 @@ usage(void)
     "    -o ignorepponrename=<bool>\n"
     "                           Ignore path preserving when performing renames\n"
     "                           and links. default = false\n"
-    "    -o threads=<int>       number of worker threads. 0 = autodetect.\n"
-    "                           Negative values autodetect then divide by\n"
-    "                           absolute value. default = 0\n"
             << std::endl;
 }