python: Drop ${PYTHON_PN}

python 2 is gone and we don't need the abstraction now, drop the remaining usage
of this variable.

The definition in python3-dir.bbclass is left for now for other layers.

(From OE-Core rev: b566b1e32c7993d1ab7795562f648e52ce186a70)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python-pbr.inc

@@ -9,6 +9,6 @@ SRC_URI += "file://0001-change-shebang-to-python3.patch"
 
 inherit pypi
 
-RDEPENDS:${PN} += "${PYTHON_PN}-pip"
+RDEPENDS:${PN} += "python3-pip"
 
 BBCLASSEXTEND = "native nativesdk"