glx/dri3: Remove unused deviceName variable

deviceName string is declared, assigned and freed but actually
never used in dri3_create_screen() function.

Fixes: 2d946015 ("Add DRI3+Present loader")
Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit d5559292)