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

[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 do, it should be separate
patch probably. The new output looks better and closer to GNU anyways,
and the code is a bit simpler.

Differential revision: https://reviews.llvm.org/D71835