Remove spurious -g causing excess rebuilds when building alr

[mosteo]

Passing -g to gprbuild via the Builder package resulted in -g being passed to all dependencies, which in turn caused alr build to always rebuild some dependencies because they saw different switches when compiled by themselves and in the context of alr.gpr.

We can always force a full debug build if wanted by adding -g to gprbuild, so removing this from alr.gpr to get rid of those rebuilds is preferable. Also, Alire itself is always build with -g (via the Compiler package in alire_common.gpr), so this removal changes nothing in Alire's regard.