1668 remove j extension placeholder chapter (riscv#1670)

* Remove J extension placeholder

With the merge of pointer masking we don't need the J extension
placeholder anymore.

* Remove inclusion of j extension

Remove include statement of j extension

Author: wmat

src/j-st-ext.adoc

@@ -185,7 +185,6 @@ include::zfinx.adoc[]
 include::c-st-ext.adoc[]
 include::zc.adoc[]
 include::b-st-ext.adoc[]
-include::j-st-ext.adoc[]
 include::p-st-ext.adoc[]
 include::v-st-ext.adoc[]
 include::scalar-crypto.adoc[]