[fix] [cli] Fix Broker crashed by too much memory usage of pulsar tools (#20031)

### Motivation
After #15868, we allow `PULSAR_MEM` & `PULSAR_GC` to be overridden in `pulsar_tool_env.sh`.

Many users set `-Xms` to `2G` or larger in `PULSAR_MEM`, this will make the tools(such as `pulsar-admin`) cost a lot of memory, and if users execute `pulsar-admin` or another tool on the machine where the Broker is deployed, the current device will not have enough memory to allocate, resulting in a broker crash.

### Modifications

When `PULSAR_MEM` is overridden  in `pulsar_tool_env.sh`, delete parameter `-Xms`

(cherry picked from commit 4f503fd)

Author: poorbarcode

conf/pulsar_tools_env.sh

@@ -42,6 +42,19 @@
 # PULSAR_GLOBAL_ZK_CONF=
 
 # Extra options to be passed to the jvm
+# Discard parameter "-Xms" of $PULSAR_MEM, which tends to be the Broker's minimum memory, to avoid using too much
+# memory by tools.
+if [ -n "$PULSAR_MEM" ]; then
+  PULSAR_MEM_ARR=("${PULSAR_MEM}")
+  PULSAR_MEM_REWRITE=""
+  for i in ${PULSAR_MEM_ARR}
+  do
+    if [ "${i:0:4}" != "-Xms" ]; then
+       PULSAR_MEM_REWRITE="$PULSAR_MEM_REWRITE $i";
+    fi
+  done
+  PULSAR_MEM=${PULSAR_MEM_REWRITE}
+fi
 PULSAR_MEM=${PULSAR_MEM:-"-Xmx128m -XX:MaxDirectMemorySize=128m"}
 
 # Garbage collection options