[rtl] clean-up simulation-only pragmas

[stnolting]

• remove redundant sim-only pragmas in package file
• UART: exclude all textio stuff for synthesis

Different vendors use different pragams for excluding simulation-only code: https://insights.sigasi.com/tech/list-known-vhdl-metacomment-pragmas/

However, this “is this a simulation?” identifier has proven itself across multiple platforms:

  -- Check if we're inside the Matrix -------------------------------------------------------
  -- -------------------------------------------------------------------------------------------
  constant is_simulation_c : boolean := false -- seems like we're on real hardware
-- pragma translate_off
-- RTL_SYNTHESIS OFF
  or true -- this MIGHT be a simulation
-- RTL_SYNTHESIS ON
-- pragma translate_on
  ;