Remove unneeded cutout for webagg in show().

_Backend.show() is used to generate a backend_module.show() method (by
wrapping _Backend.mainloop() with some boilerplate), which is then what
pyplot.show() calls.  Currently, it contains a cutout for webagg (wrt.
`block` support), but that's not really needed: the webagg backend
defines a show() function *directly*, without going through the mainloop
helper (this was the case even before the introduction of the _Backend
helper in 5141f80), and thus the `if get_backend() == "WebAgg"` check is
never reached when using webagg (try adding a print there and running
`MPLBACKEND=webagg python -c 'from pylab import *; plot(); show()'`).
So we can just remove the cutout.

Author: anntzer

lib/matplotlib/backend_bases.py

@@ -3507,10 +3507,6 @@ def show(cls, *, block=None):
             except AttributeError:
                 ipython_pylab = False
             block = not ipython_pylab and not is_interactive()
-            # TODO: The above is a hack to get the WebAgg backend working with
-            # ipython's `%pylab` mode until proper integration is implemented.
-            if get_backend() == "WebAgg":
-                block = True
         if block:
             cls.mainloop()