Avoid to force PORTABLE mode in tools/regression_test.sh (#7806)

Summary:
Right now tools/regression_test.sh always builds RocksDB with PORTABLE=1. There isn't a reason for that. Remove it. Users can always specify PORTABLE through  envirionement variable.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7806

Test Plan: Run tools/regression_test.sh and see it still builds.

Reviewed By: ajkr

Differential Revision: D25687911

fbshipit-source-id: 1c0b03e5df890babc8b7d8af48b48774d9a4600c

tools/regression_test.sh

@@ -377,7 +377,7 @@ function build_db_bench_and_ldb {
   make clean
   exit_on_error $?
 
-  DEBUG_LEVEL=0 PORTABLE=1 make db_bench ldb -j32
+  DEBUG_LEVEL=0 make db_bench ldb -j32
   exit_on_error $?
 }