zebra: Remove dead SRv6 code

zebra: Remove dead SRv6 code

At line 1736, `alloc_mode` is set to `SRV6_SID_ALLOC_MODE_EXPLICIT` or
`SRV6_SID_ALLOC_MODE_DYNAMIC` depending on the `sid_value` variable.

There will never be a case where alloc_mode will be `SRV6_SID_ALLOC_MODE_MAX`
or `SRV6_SID_ALLOC_MODE_UNSPEC`.

Let's replace the `switch(alloc_mode) {...}` with an if-else.

Fixes CID 1594015.

** CID 1594015:    (DEADCODE)
/zebra/zebra_srv6.c: 1782 in get_srv6_sid()
/zebra/zebra_srv6.c: 1781 in get_srv6_sid()

________________________________________________________________________________________________________
*** CID 1594015:    (DEADCODE)
/zebra/zebra_srv6.c: 1782 in get_srv6_sid()
1776     		}
1777
1778     		ret = get_srv6_sid_dynamic(sid, ctx, locator);
1779
1780     		break;
1781     	case SRV6_SID_ALLOC_MODE_MAX:
     CID 1594015:    (DEADCODE)
     Execution cannot reach this statement: "case SRV6_SID_ALLOC_MODE_UN...".
1782     	case SRV6_SID_ALLOC_MODE_UNSPEC:
1783     	default:
1784     		flog_err(EC_ZEBRA_SM_CANNOT_ASSIGN_SID,
1785     			 "%s: SRv6 Manager: Unrecognized alloc mode %u",
1786     			 __func__, alloc_mode);
1787     		/* We should never arrive here */
/zebra/zebra_srv6.c: 1781 in get_srv6_sid()
1775     			return -1;
1776     		}
1777
1778     		ret = get_srv6_sid_dynamic(sid, ctx, locator);
1779
1780     		break;
     CID 1594015:    (DEADCODE)
     Execution cannot reach this statement: "case SRV6_SID_ALLOC_MODE_MAX:".
1781     	case SRV6_SID_ALLOC_MODE_MAX:
1782     	case SRV6_SID_ALLOC_MODE_UNSPEC:
1783     	default:
1784     		flog_err(EC_ZEBRA_SM_CANNOT_ASSIGN_SID,
1785     			 "%s: SRv6 Manager: Unrecognized alloc mode %u",
1786     			 __func__, alloc_mode);

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>