Revert "Enabling Developer Toolset 7 for Jenkins' lo_tb_random_config_linux"

This reverts commit 4d72f292.  Developer
Toolset 7 has apparently not yet been installed on gandalf (the node exclusively
executing this job), see
<https://ci.libreoffice.org/job/lo_tb_random_config_linux/1623/console>:

> checking for gcc... /opt/rh/devtoolset-7/root/usr/bin/gcc
> checking whether the C compiler works... no
> configure: error: in `/lo/home/tdf/lode/jenkins/workspace/lo_tb_random_config_linux':
> configure: error: C compiler cannot create executables
> See `config.log' for more details
> Error running configure at ./autogen.sh line 296.

distro-configs/Jenkins/linux_random.conf

 --enable-fuzz-options
-CC=/opt/rh/devtoolset-7/root/usr/bin/gcc
-CXX=/opt/rh/devtoolset-7/root/usr/bin/g++