[llvm-readobj] - Remove an excessive helper for printing dynamic tags.

This removes the `getTypeString` from readeobj source because it
almost duplicates the existent method: `ELFFile<ELFT>::getDynamicTagAsString`.

Side effect: now it prints "<unknown:>0xHEXVALUE" instead of "(unknown)" for unknown values.
llvm-readelf before this patch printed:

```
0x0000000012345678 (unknown) 0x8765432187654321
0x000000006abcdef0 (unknown) 0x9988776655443322
0x0000000076543210 (unknown) 0x5555666677778888
```

and now it prints:

```
0x0000000012345678 (<unknown:>0x12345678) 0x8765432187654321
0x000000006abcdef0 (<unknown:>0x6abcdef0) 0x9988776655443322
0x0000000076543210 (<unknown:>0x76543210) 0x5555666677778888
```

GNU reaedlf prints different thing:

```
0x0000000012345678 (<unknown>: 12345678) 0x8765432187654321
0x000000006abcdef0 (Operating System specific: 6abcdef0) 0x9988776655443322
0x0000000076543210 (Processor Specific: 76543210) 0x5555666677778888
```

I am not sure we want to follow GNU here. Even if we d...