[mlir][ods] Remove StrEnumAttr

[mlir][ods] Remove StrEnumAttr

StrEnumAttr has been deprecated in favour of EnumAttr, a solution based on AttrDef (https://reviews.llvm.org/D115181). This patch removes StrEnumAttr, along with all the custom ODS logic required to handle it.

See https://discourse.llvm.org/t/psa-stop-using-strenumattr-do-use-enumattr/5710 on how to transition to EnumAttr. In short,

```
// Before
def MyEnumAttr : StrEnumAttr<"MyEnum", "", [
  StrEnumAttrCase<"A">,
  StrEnumAttrCase<"B">
]>;

// After (pick an integer enum of your choice)
def MyEnum : I32EnumAttr<"MyEnum", "", [
  I32EnumAttrCase<"A", 0>,
  I32EnumAttrCase<"B", 1>
]> {
  // Don't generate a C++ class! We want to use the AttrDef
  let genSpecializedAttr = 0;
}
// Define the AttrDef
def MyEnum : EnumAttr<MyDialect, MyEnum, "my_enum">;
```

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D120834