contrib/jenkins.sh: actually build with $PARALLEL_MAKE

Parallel build has been fixed [1] and re-enabled [2] back in 2018.

Executing $MAKE without $PARALLEL_MAKE builds the project with -j1.
Executing $MAKE with $PARALLEL_MAKE after that makes no sense,
because everything is already compiled and there is nothing to do.

Change-Id: Iff7e7acfac0b827ce3b1086cea376a438b5af334
Related: [1] I5a9d7dbd7b992d322ed0d852ebf8ca2252b51a12
Related: [2] Id41fbcb5a96093eb6c3dc00bcacbd379111ada70
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 3b4e639..7a75e25 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -7,7 +7,6 @@
 
 autoreconf --install --force
 ./configure --enable-sanitize --enable-werror
-$MAKE
 $MAKE $PARALLEL_MAKE
 $MAKE distcheck || cat-testlogs.sh
 $MAKE maintainer-clean