rename property "kIsFileDeletionsEnabled" to "kIsFileDeletionsDisabled" (#9791)

Summary:
The name of this property "kIsFileDeletionsEnabled" is very, very easy to misunderstand.

I think 0 represents false (i.e. disabled) and non-0 means true (enabled), and this property is just the opposite.

I modified the name of this property, and as few other positions as possible, so that the final meaning remains the same, but the name of this property is more common sense.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9791

Reviewed By: ajkr

Differential Revision: D35362166

Pulled By: jay-zhuang

fbshipit-source-id: 85310d88bdd131893effb64e1adb7d0d7b202f88

include/rocksdb/db.h

@@ -924,6 +924,8 @@ class DB {
 
     //  "rocksdb.is-file-deletions-enabled" - returns 0 if deletion of obsolete
     //      files is enabled; otherwise, returns a non-zero number.
+    //  This name may be misleading because true(non-zero) means disable,
+    //  but we keep the name for backward compatibility.
     static const std::string kIsFileDeletionsEnabled;
 
     //  "rocksdb.num-snapshots" - returns number of unreleased snapshots of the