build: remove the bundled readline/ subdirectory (Patch 3)

Delete readline/ (the vendored GNU readline 4.3, ~104 files / 2 MB) now that
configure requires the system readline (Patch 1) and the Makefiles no longer
wire the bundled build (Patch 2).  Sweep the remaining references:

  * commands/Makefile.in: drop the `module: ${MAGICDIR}/readline/readline` rule
    and the readline-create-symlinks delegation (bundled-only; commands now uses
    the default `module: libcommands.o`).
  * commands/CmdFI.c, textio/txMain.c, textio/txInput.c: the readline includes
    had a `#ifdef HAVE_READLINE <system> #else "readline/readline/*.h" (bundled)`
    split.  HAVE_READLINE/NEED_READLINE are used nowhere else and USE_READLINE now
    always implies the system library, so collapse to the plain
    `#include <readline/readline.h>` / `<history.h>`.
  * CI: main.yml's non-Tcl job installs libreadline-dev (was relying on bundled);
    canary-matrix drops the now-invalid no_tk_tcl_brl / --enable-readline-bundled
    variant.
  * .gitignore: drop the "readline/ intentionally not converted" note.

Verified: Tcl -j12 builds+installs 375 files (commands builds, no readline-dir
references); --without-tcl --disable-readline -j12 builds (x2); --without-tcl with
no readline dev package still errors with the guidance message.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

commands/CmdFI.c

@@ -25,13 +25,8 @@ static const char rcsid[] __attribute__ ((unused)) = "$Header: /usr/cvsroot/magi
 #include <string.h>
 
 #ifdef USE_READLINE
-#ifdef HAVE_READLINE
 #include <readline/readline.h>
 #include <readline/history.h>
-#else
-#include "readline/readline/readline.h"
-#include "readline/readline/history.h"
-#endif
 #endif
 
 #include "tcltk/tclmagic.h"

commands/Makefile.in

@@ -11,14 +11,5 @@ MAGICDIR  = @top_builddir@
 SRCS      = CmdSubrs.c CmdAB.c CmdCD.c CmdE.c CmdFI.c \
             CmdLQ.c CmdRS.c CmdTZ.c CmdWizard.c CmdAuto.c
 
-# Force the module to regenerate the symbolic link in the readline
-# directory, as it may be needed by CmdFI.c (in the non-Tcl compile)
-
-module: ${MAGICDIR}/readline/readline lib${MODULE}.o
-
-# Delegate this task to the readline/Makefile
-${MAGICDIR}/readline/readline:
-	${MAKE} -C ${MAGICDIR}/readline readline-create-symlinks
-
 include ${MAGICDIR}/defs.mak
 include ${MAGICSRC}/rules.mak