readme.md: remove mention of Xbootclasspath
readme.md: remove mention of Xbootclasspath

It isn't required to run IDE or tests anymore (IDEA-185591).

allotria/intellij-community,allotria/intellij-community,allotria/intellij-community,xfournet/intellij-community,xfournet/intellij-community,xfournet/intellij-community,allotria/intellij-community,xfournet/intellij-community,allotria/intellij-community,xfournet/intellij-community,xfournet/intellij-community,allotria/intellij-community,xfournet/intellij-community,allotria/intellij-community,xfournet/intellij-community,allotria/intellij-community,xfournet/intellij-community,xfournet/intellij-community,allotria/intellij-community,allotria/intellij-community,allotria/intellij-community,xfournet/intellij-community,allotria/intellij-community,allotria/intellij-community,xfournet/intellij-community,xfournet/intellij-community