package/python-matplotlib: drop redundant !PYTHON dependency

Having both:

       depends on !BR2_PACKAGE_PYTHON
       depends on BR2_PACKAGE_PYTHON3

is kind of useless, and we don't do that for any other Python package
that needs Python 3. So, this commit just drops the depends on
!BR2_PACKAGE_PYTHON.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>