Test always evaluates the same

Defect 77704 (STATIC_C)
Checker DEADCODE (subcategory redundant_test)
File: /src/third_party/wiredtiger/src/schema/schema_create.c
Function __create_index
/src/third_party/wiredtiger/src/schema/schema_create.c, line: 553
At condition "exists", the value of "exists" must be equal to 0.

    	if (!exists)

/src/third_party/wiredtiger/src/schema/schema_create.c, line: 403
Assigning: "exists" = "have_extractor = false".

    	exists = have_extractor = false;

/src/third_party/wiredtiger/src/schema/schema_create.c, line: 553
The condition "!exists" must be true.

    	if (!exists)