Fix bindep.txt: clean remaining python3.7

While upgrading tox37 to tox38, those lines were not updated/removed
they can be removed as:
-tox-py3X jobs specify python version to install during ensure-python
-other jobs are most likely using ensure-pip, which on ubuntu-bionic
install python3-venv and python3.6

Change-Id: I0abfa76d4a7057c8b9154cc111bb0aa20f4fe0bb

bindep.txt

@@ -37,8 +37,6 @@ gmp [platform:apk]
 procps [platform:apk platform:dpkg]
 python3-dev [compile test platform:dpkg platform:apk]
 python3-devel [compile test platform:rpm]
-python3.7 [test platform:ubuntu-bionic]
-python3.7-dev [test platform:ubuntu-bionic]
 bubblewrap [!platform:ubuntu-xenial]
 socat
 libre2-dev [compile test platform:dpkg]