bpo-43290: Remove workaround from pysqlite_step() (GH-24638)

From the SQLite 3.5.3 changelog:

sqlite3_step() returns SQLITE_MISUSE instead of crashing when called
with a NULL parameter.

The workaround no longer needed because we no longer support
SQLite releases older than 3.7.15.

Modules/_sqlite/util.c

@@ -28,15 +28,9 @@ int pysqlite_step(sqlite3_stmt* statement, pysqlite_Connection* connection)
 {
     int rc;
 
-    if (statement == NULL) {
-        /* this is a workaround for SQLite 3.5 and later. it now apparently
-         * returns NULL for "no-operation" statements */
-        rc = SQLITE_OK;
-    } else {
-        Py_BEGIN_ALLOW_THREADS
-        rc = sqlite3_step(statement);
-        Py_END_ALLOW_THREADS
-    }
+    Py_BEGIN_ALLOW_THREADS
+    rc = sqlite3_step(statement);
+    Py_END_ALLOW_THREADS
 
     return rc;
 }