Remove redundant conditional in julia-launcher

[PranavGadge01]

Problem

The julia-launcher function in progs/init-julia.scm contained
an if/else where both branches produced identical output:

(define (julia-launcher)
  (with boot (raw-quote (julia-entry))
    (if (url-exists-in-path? "julia")
        (string-append "julia " boot)
        (string-append "julia " boot))))  ; <- identical to true branch

This conditional is dead code — it never influences the return value
regardless of whether Julia is found or not.

Additionally, the false branch is structurally unreachable. The
:require (url-exists-in-path? "julia") clause in plugin-configure
runs before the launcher is ever invoked. If Julia is absent from PATH,
TeXmacs disables the plugin entirely at that point and never calls
julia-launcher.

Fix

Simplify julia-launcher to a single expression:

(define (julia-launcher)
  (with boot (raw-quote (julia-entry))
    (string-append "julia " boot)))

Impact

No runtime behaviour change. This is a code clarity fix — the
previous code falsely implied error handling logic was present
when it was not.

[PranavGadge01]

@hammerfunctor could u please check...