First, you should not need to uninstall Java 8 to switch to Java 7, due to the alternatives system. Merely installing oracle-java7-installer will make Java 7 the preferred JVM/JDK. If you ever do need Java 8:
$ sudo update-java-alternatives --set java-8-oracle
Note that this changes /usr/bin/java, thus affecting everything on that system that relies on the default java. You may want to set JAVA_HOME (to, e.g., /usr/lib/jvm/java-8-oracle) if there is a single app that needs a different java.
WARNING: This works only for Oracle Java installed via webupd8 packages! Adding any OpenJDK package has undefined results!
If you really do need to get rid of Java8, the removal scripts for oracle-java8-installer will remove the JDK as well as the installer.