boards: bflb: remove redundant file reference

Board documentation page already has a link to the board's folder in
GitHub so hardcoding a link to the board's defconfig is redundant.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

boards/bflb/bl60x/bl604e_iot_dvk/doc/index.rst

@@ -34,9 +34,6 @@ Supported Features
 
 .. zephyr:board-supported-hw::
 
-The default configuration can be found in the Kconfig
-:zephyr_file:`boards/bouffalolab/bl60x/bl604e_iot_dvk/bl604e_iot_dvk_defconfig`.
-
 System Clock
 ============