doc: Remove attic classes from Doxygen (#12795)

When attic modules are shown in the C++ documentation, there is noting that
highlights them as legacy or to-be-deprecated so it is too easy for users to
stumble into misleading information.

(Note that for pydrake, attic code lives in the attic module so at least the
information is segregated.)

doc/Doxyfile_CXX.in

@@ -135,7 +135,7 @@ FILE_PATTERNS          = *.c \
 RECURSIVE              = YES
 EXCLUDE                =
 EXCLUDE_SYMLINKS       = NO
-EXCLUDE_PATTERNS       = "@INPUT_ROOT@/*/dev/*"
+EXCLUDE_PATTERNS       = "@INPUT_ROOT@/*/dev/*" "@INPUT_ROOT@/drake/attic/*"
 EXCLUDE_SYMBOLS        =
 EXAMPLE_PATH           =
 EXAMPLE_PATTERNS       =