Revert "doc: enable figures enumeration" cause numfig is too slow

This reverts commit 4516117.

A git bisect showed that the duration of an incremental build doubled
after this commit enabled `numfig=True`. Measurements shared and
discussed in zephyrproject-rtos#37572, zephyrproject-rtos#55708 and zephyrproject-rtos#56631 confirmed this. Here are yet more
measurements below in two different system configurations building docs
for very recent Zephyr commit b10817b + all `:numref:` removed by
the previous commit. In other words these numbers show the cost of
`numfig=True` _without_ even using `:numref:`.

* Ubuntu 22, 8 cores
  sphinx-build --version 4.3.2

                           numfig=True   numfig=False

- from scratch             7 min 15 s       6 min 40 s

- one-line .rst change           48 s             24s

* Current Arch Linux, 72 cores
  sphinx-build --version 6.2.1

                           numfig=True   numfig=False

- from scratch              5 min 0 s      4 min 50 s

- one-line .rst change           37 s            18 s

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

doc/conf.py

@@ -103,8 +103,6 @@
 
 todo_include_todos = False
 
-numfig = True
-
 nitpick_ignore = [
     # ignore C standard identifiers (they are not defined in Zephyr docs)
     ("c:identifier", "FILE"),