std.math: remove constants that should be expressions

All four of these can be represented in fewer characters with
expressions, which will be guaranteed to happen at compile-time, and
have the same or better precision.

The other math constants here which depend on function calls could be
similarly removed if and when #425 is solved. However I left them for
now since Zig does not eagerly evaluate functions with comptime parameters.

lib/std/math.zig

@@ -25,18 +25,6 @@ pub const ln2 = 0.693147180559945309417232121458176568;
 /// ln(10)
 pub const ln10 = 2.302585092994045684017991454684364208;
 
-/// π/2
-pub const pi_2 = 1.570796326794896619231321691639751442;
-
-/// π/4
-pub const pi_4 = 0.785398163397448309615660845819875721;
-
-/// 1/π
-pub const one_pi = 0.318309886183790671537767526745028724;
-
-/// 2/π
-pub const two_pi = 0.636619772367581343075535053490057448;
-
 /// 2/sqrt(π)
 pub const two_sqrtpi = 1.128379167095512573896158903121545172;