[Misc] Remove unnecessary clean up before build to accelerate incremental build. (#5267)

* remove unnecessary clean up.

* more

---------
Co-authored-by: Steve <ubuntu@ip-172-31-34-29.ap-northeast-1.compute.internal>

dgl_sparse/build.sh

@@ -2,7 +2,6 @@
 # Helper script to build dgl sparse libraries for PyTorch
 set -e
 
-rm -rf build
 mkdir -p build
 mkdir -p $BINDIR/dgl_sparse
 cd build
@@ -24,7 +23,6 @@ if [ $# -eq 0 ]; then
 	cp -v $CPSOURCE $BINDIR/dgl_sparse
 else
 	for PYTHON_INTERP in $@; do
-		rm -rf *
 		$CMAKE_COMMAND $CMAKE_FLAGS -DPYTHON_INTERP=$PYTHON_INTERP ..
 		make -j
 		cp -v $CPSOURCE $BINDIR/dgl_sparse

tensoradapter/pytorch/build.sh

@@ -2,7 +2,6 @@
 # Helper script to build tensor adapter libraries for PyTorch
 set -e
 
-rm -rf build
 mkdir -p build
 mkdir -p $BINDIR/tensoradapter/pytorch
 cd build
@@ -21,7 +20,6 @@ if [ $# -eq 0 ]; then
 	cp -v $CPSOURCE $BINDIR/tensoradapter/pytorch
 else
 	for PYTHON_INTERP in $@; do
-		rm -rf *
 		$CMAKE_COMMAND $CMAKE_FLAGS -DPYTHON_INTERP=$PYTHON_INTERP ..
 		make -j
 		cp -v $CPSOURCE $BINDIR/tensoradapter/pytorch