Revert "time: add weak annotation to timer_read_counter declaration"

This reverts commit 65ba7add.

A weak extern is a nasty sight to behold: If the symbol is never
defined, on ARM, the linker will replace the function call with a NOP.
This behavior isn't well documented but there are at least some hints
to it [1].

When timer_read_counter() is not defined, this obviously does the wrong
thing here and it does so silently.  The consequence is that a board
without timer_read_counter() will sleep for random amounts and generally
have erratic get_ticks() values.

Drop the __weak annotation of the extern so a linker error is raised
when timer_read_counter() is not defined.  This is okay, the original
reason for the reverted change - breaking the sandbox build - no longer
applies.

Final sidenote:  This was the only weak extern in the entire tree at
this time as far as I can tell.  I guess we should avoid introduction of
them again as they are obviously a very big footgun.

[1]: https://stackoverflow.com/questions/31203402/gcc-behavior-for-unresolved-weak-functions



Fixes: 65ba7add ("time: add weak annotation to timer_read_counter declaration")
Reported-by: Serge Bazanski <q3k@q3k.org>
Signed-off-by: Harald Seiler <hws@denx.de>