I don't think that uninstalling packages is mandatory but if you want to do it anyways, here is the command remove.packages('rWeka')
Usually the problem with any packages that requires rJava is making rstudio to recognise your java installation path, on windows I have managed to make it work by modifying the JAVA_HOME variable from my Rprofile.site file by adding this line to it:
Sys.setenv('JAVA_HOME'="C:/Program Files/Java/jdk-11.0.1/") Obviously the path has to be modified to fit your own setup.