Remove the obsolete mkfont.c and font_10x18.h

The font_10x18.h is used as a fall back for fonts if the
$(recovery_font) under bootable/recovery/fonts failed to reload. This
font looks very small on the screen and we haven't used it in a while.
Plus, fonts in header doesn't have many benefits if we support only
ascii characters. So we decide to remove this header along with the mkfont.c
that generates the header.

For reference, here's the steps to use mkfont:
1. Generate a picture of fonts with apps on host
2. Open the pic with GIMP
3. Export the pic as '.c' file, and rename it into '.h' file
4. Compile the generated '.h' file with mkfont.c

Bug: 76420958
Test: build and check the recovery menu on angler
Change-Id: I6c108aa43a07bf4994115ef764275bd84df725e1