Bug 1686333: don't install `libpython2-dev` r=firefox-build-system-reviewers,mhentges

Seems it isn't always available. Since Py2 is optional anyways, it
shouldn't cause failures in `mach bootstrap`.

Differential Revision: https://phabricator.services.mozilla.com/D101499

python/mozboot/mozboot/debian.py

@@ -32,7 +32,6 @@ class DebianBootstrapper(LinuxBootstrapper, BaseBootstrapper):
     # Ubuntu).
     COMMON_PACKAGES = [
         "build-essential",
-        "libpython2-dev",
         "libpython3-dev",
         "nodejs",
         "unzip",