linuxKernel: drop conditionals for versions greater Linux 4.16; that's all of them

pkgs/os-specific/linux/kernel/generic.nix

@@ -140,8 +140,7 @@ let
     passAsFile = [ "kernelConfig" ];
 
     depsBuildBuild = [ buildPackages.stdenv.cc ];
-    nativeBuildInputs = [ perl gmp libmpc mpfr ]
-      ++ lib.optionals (lib.versionAtLeast version "4.16") [ bison flex ]
+    nativeBuildInputs = [ perl gmp libmpc mpfr bison flex ]
       ++ lib.optional (lib.versionAtLeast version "5.2") pahole
       ++ lib.optionals withRust [ rust-bindgen rustc ]
     ;

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -143,6 +143,8 @@ let
 
       depsBuildBuild = [ buildPackages.stdenv.cc ];
       nativeBuildInputs = [
+        bison
+        flex
         perl
         bc
         nettools
@@ -156,7 +158,6 @@ let
         python3Minimal
         kmod
       ] ++ optional  needsUbootTools ubootTools
-        ++ optionals (lib.versionAtLeast version "4.16") [ bison flex ]
         ++ optionals (lib.versionAtLeast version "5.2")  [ cpio pahole zlib ]
         ++ optionals withRust [ rustc rust-bindgen ];