ceph.spec.in: drop the python3_pkgversion macro

ceph.spec.in: drop the python3_pkgversion macro

python3_pkgversion picked the native python3 package name on RHEL7, where
python3 was python34/python36 before 7.6 (cea9d18c). RHEL7 and RHEL8 are
gone, so on every supported distro (el9, el10, fedora, suse, openEuler) it
is always 3.

Replace python%{python3_pkgversion} with python3 and drop the fallback
default. No supported distro overrides it to build against a non-default
python flavor, so that capability goes too.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>