asymptote: remove display side-effect

When building the documentation, the call to asymptote outputs the pwd
and the checksum; this interrupts the new progress bar mechanism in
recent Racket versions. Silence it by removing the display entirely.

(Perhaps we should provide a function to get the hash or document how it
is computed? The function could also accept a side-channel to
communicate the hash to, such as a box. Modifying it to return multiple
values would be a breaking change.)

asymptote.rkt

@@ -23,8 +23,6 @@
              [pdf-path (build-path asymptote-dir pdf-name)]
              [svg-name (string-append md ".svg")]
              [svg-path (build-path asymptote-dir svg-name)])
-        (display (current-directory)) (display md) (newline)
-
         ;; create dir if neccessary
         (unless (directory-exists? asymptote-dir)
           (make-directory asymptote-dir))