Fix string to float conversion when typing gain values in Gain Meter

Fix string to float conversion when typing gain values in Gain Meter

Remove the LocaleGuard so that the value of LC_NUMERIC is that of the users
locale and sscanf will parse the string correctly. For instance, with a locale
like nl_NL or fr_FR where the decimal point is different than the "C" locale,
only the number up to the decimal point will be parsed by sscanf and input
values will be rounded down.