fix: left index of system type can't be deleted

[javeme]

Change-Id: I654b2e4c69f8aebfc7ebed3225b2dd09da1720a0

[codecov]

Codecov Report

Merging #675 into master will decrease coverage by 0.05%.
The diff coverage is 46.87%.

@@             Coverage Diff              @@
##             master     #675      +/-   ##
============================================
- Coverage        73%   72.94%   -0.06%     
  Complexity     3944     3944              
============================================
  Files           219      219              
  Lines         17850    17867      +17     
  Branches       2567     2569       +2     
============================================
+ Hits          13031    13033       +2     
- Misses         3479     3492      +13     
- Partials       1340     1342       +2

Impacted Files	Coverage Δ	Complexity Δ
...aidu/hugegraph/backend/store/mysql/MysqlStore.java	79.66% <ø> (+0.44%)	37 <0> (ø)	⬇️
...ugegraph/backend/store/memory/InMemoryDBStore.java	85.15% <ø> (+0.66%)	24 <0> (ø)	⬇️
...egraph/backend/store/cassandra/CassandraStore.java	69.4% <ø> (+0.25%)	48 <0> (ø)	⬇️
...java/com/baidu/hugegraph/schema/SchemaElement.java	76.92% <0%> (-3.08%)	21 <1> (+1)
.../hugegraph/backend/store/AbstractBackendStore.java	71.42% <0%> (-5.5%)	3 <0> (ø)
...e/src/main/java/com/baidu/hugegraph/HugeGraph.java	71.55% <42.85%> (-1.89%)	67 <1> (+1)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	80.31% <85.71%> (-0.06%)	176 <1> (ø)
...du/hugegraph/backend/store/BackendSessionPool.java	75.8% <0%> (-8.07%)	15% <0%> (-2%)
...a/com/baidu/hugegraph/backend/query/Condition.java	77.59% <0%> (+0.41%)	31% <0%> (ø)	⬇️

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[Linary]

Does the comment and the code have some inconsistencies?

this.tx.readWrite();