In pgf_preamble_sgskip, replace examples of using units and metalogo
package by url, which is in TeXLive latex-base (unlike the others) and
thus most likely to be present (also, the url works in the pdf, which is
nice). Remove dependency on xits-math.otf font.

Sync the description of text.latex.preamble in classic.mplstyle with the
current template description. This removes the substrings
"\usepackage{bm},\usepackage{euler}", which caused some linux packagers
in adding dependencies on these packages; moreover, the comma is now
wrong.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).