Remove RHEL 8 rules for several Python 2 keys (ros#24690)

Though RHEL 8 contains a select few Python 2 packages right now, it
probably won't pick up any new ones, so these rules are never expected
to be correct looking forward from RHEL 7.

rosdep/python.yaml

@@ -231,7 +231,8 @@ paramiko:
  osx:
  pip:
  packages: [paramiko]
- rhel: [python-paramiko]
+ rhel:
+ '7': [python-paramiko]
  ubuntu: [python-paramiko]
 pi-ina219-pip:
  debian:
@@ -2072,7 +2073,8 @@ python-imaging:
  osx:
  pip:
  packages: [Pillow]
- rhel: [python-imaging]
+ rhel:
+ '7': [python-imaging]
  slackware:
  slackpkg:
  packages: [python-pillow]
@@ -2384,7 +2386,8 @@ python-matplotlib:
  pip:
  depends: [pkg-config, freetype, libpng12-dev]
  packages: [matplotlib]
- rhel: [python-matplotlib]
+ rhel:
+ '7': [python-matplotlib]
  slackware: [matplotlib]
  ubuntu:
  artful: [python-matplotlib]
@@ -2886,7 +2889,8 @@ python-paramiko:
  osx:
  pip:
  packages: [paramiko]
- rhel: [python-paramiko]
+ rhel:
+ '7': [python-paramiko]
  slackware: [paramiko]
  ubuntu: [python-paramiko]
 python-parse:
@@ -3853,7 +3857,8 @@ python-rosdep-modules:
  osx:
  pip:
  packages: [rosdep]
- rhel: ['python-rosdep']
+ rhel:
+ '7': [python-rosdep]
  slackware:
  pip:
  packages: [rosdep]