Remove serdect by removing the unnecessary "alloc" feature from crypto-bigint

This only works for the legacy `crypto-bigint` and downstream consumers who
don't have the modern `serdect` pulled in for independent reasons.

Cargo.lock

@@ -2316,7 +2316,7 @@ checksum = "0dc92fb57ca44df6db8059111ab3af99a63d5d0f8375d9972e319a379c6bab76"
 dependencies = [
  "generic-array 0.14.7",
  "rand_core 0.6.4",
- "serdect 0.2.0",
+ "serdect",
  "subtle",
  "zeroize",
 ]
@@ -2328,7 +2328,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "96272c2ff28b807e09250b180ad1fb7889a3258f7455759b5c3c58b719467130"
 dependencies = [
  "num-traits",
- "serdect 0.3.0",
  "subtle",
  "zeroize",
 ]
@@ -2943,7 +2942,7 @@ dependencies = [
  "digest 0.10.7",
  "elliptic-curve",
  "rfc6979",
- "serdect 0.2.0",
+ "serdect",
  "signature",
  "spki",
 ]
@@ -3023,7 +3022,7 @@ dependencies = [
  "pkcs8",
  "rand_core 0.6.4",
  "sec1",
- "serdect 0.2.0",
+ "serdect",
  "subtle",
  "tap",
  "zeroize",
@@ -4761,7 +4760,7 @@ dependencies = [
  "ecdsa",
  "elliptic-curve",
  "once_cell",
- "serdect 0.2.0",
+ "serdect",
  "sha2 0.10.9",
 ]
 
@@ -9561,7 +9560,7 @@ dependencies = [
  "der",
  "generic-array 0.14.7",
  "pkcs8",
- "serdect 0.2.0",
+ "serdect",
  "subtle",
  "zeroize",
 ]
@@ -10917,16 +10916,6 @@ dependencies = [
  "serde",
 ]
 
-[[package]]
-name = "serdect"
-version = "0.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f42f67da2385b51a5f9652db9c93d78aeaf7610bf5ec366080b6de810604af53"
-dependencies = [
- "base16ct",
- "serde",
-]
-
 [[package]]
 name = "sha-1"
 version = "0.9.8"