chore(io): drop global Abaqus keyword registration helpers

Remove the mutable global register used by the retired `.inp` parser to tag
top-level keywords.

- Delete `register_abaqus_keyword` / `is_abaqus_keyword_registered` from
  `src/io/keyword_register.jl` (Legacy carries its own registration path).

src/io/keyword_register.jl

@@ -1,24 +0,0 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/AbaqusReader.jl/blob/master/LICENSE
-
-global __register__ = Set{String}()
-
-"""
-    register_abaqus_keyword(keyword::String)
-
-Add ABAQUS keyword `s` to register. That is, after registration every time
-keyword show up in `.inp` file a new section is started
-"""
-function register_abaqus_keyword(keyword::String)
-    push!(__register__, keyword)
-    return Type{Val{Symbol(keyword)}}
-end
-
-"""
-    is_abaqus_keyword_registered(keyword::String)
-
-Return true/false is ABAQUS keyword registered.
-"""
-function is_abaqus_keyword_registered(keyword::String)
-    return keyword in __register__
-end