[gdb/tui] Simplify tui_enable

I noticed some code in tui_enable doing:
...
if (...)
  error (...);
else if (...)
...
which is the "Don’t use else after a return" anti-pattern [1].

Fix this by using:
...
if (...)
  error (...);

if (...)
...

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://llvm.org/docs/CodingStandards.html#id41